Comprehensive natural gas processing

ABSTRACT

The present invention related to a process and an apparatus for efficient and cost-effective comprehensive processing of natural gas, including the removal of moisture and the recovery of the higher hydrocarbons components (C 2   + ). The said apparatus comprises the following major components: an integrated natural gas processor with a dehydration section and a higher hydrocarbons absorption section; a heat transport medium cooler; an absorbent cooler; a fractional distiller for separating the light oil from the heavy oil absorbent; an inhibitor regenerator; and a refrigeration unit. The present invention provides a low-cost natural gas comprehensive processor  processing that is universally applicable to both terrestrial and off-shore natural gas exploitation. The said apparatus also provides an efficient and cost-effective natural gas dehydrator when the dehydration section is used independently without incorporating the absorption section.

BACKGROUND OF INVENTION

The reduction of CO₂ emission is one of the greatest concerns incombating the catastrophic “global warming” trend. As a result, theworld puts much emphasis on the exploitation of “clean energy” with lessor non-emission for both industrial and domestic uses. Natural gas(hereafter abbreviated as “NG”), as compared with coal and petroleum, isconsidered the most economic “clean” fuel that is used on a large,industrial scale at present and in the near future. In addition, thediscovery of huge amount of ocean-bed gas-hydrates increases therecoverable resources of NG substantially. It is expected that, in thelong run, the global NG consumption may eventually exceeds all otherfossil fuels.

NG is a mixture of hydrocarbon gases, consisting of mainly methane (C₁)and a smaller fraction of heavier gaseous hydrocarbons (i.e., ethane,C₂; propane, C₃; butane, C₄; pentane and higher, C₅ ⁺; sometimes C₃+ iscalled “light oil” as a whole. However, the economic values of thesehigher hydrocarbon components, when separated and sold as chemicalfeedstock, are usually much higher than burnt as a fuel. A number of NGprocessing plants, therefore, have been constructed to extract thesevaluable materials.

The state-of-the-art NG processing plants generally work on a cryogenicprocess for efficiently separating the higher hydrocarbon gases In thisprocess, a huge volume of NG is cooled down by expansion to a very lowcryogenic temperature around −150° F. Such a process is extremelyenergy-consuming, and the facility usually comprises many pieces ofexpensive equipment, notably the molecular-sieve dehydrator, themultiple-flow finned-plate heat exchanger, and the turboexpander-compressor. High capital and operational costs are thusresulted. As a consequence, only a limited fraction of the NG could beprocessed before consumed as a fuel. Most of the valuable higherhydrocarbon contents was improperly used.

In the past two decades, a number of US patents have been granted inthis field, for example, the 13 US patents entitled “hydrocarbonProcessing” presented by late Roy E. Campbell, et al., i.e., U.S. Pat.Nos. 4,140,504; 4,157,904; 4,171,964; 4,278,457; 4,854,955; 4,869,740;4,889,545; 5,555,784; 5,568,737; 5,771,712; 5,881,569; 5,983,664; and6,182,469. However, most of these patents only proposed some specificimprovements to the same cryogenic process. No substantial break-throughin NG processing technology has ever been proposed. A more efficient andcost-effective technology for NG procession, therefore, is desirable.

The recent developments in NG refrigeration dehydration technology,e.g., those presented in U.S. Pat. No. 5,664,426, “Regenerative GasDehydrator,” 1997, and U.S. Pat. No. 6,158,242, “Gas Dehydration Methodand Apparatus,” 2000, provided the basis of a break-through in the NGprocessing technology. These patents make possible to performrefrigeration dehydration and refrigeration absorption in a single unit.

Accordingly, it is an objective of the present invention to provide acomprehensive NG process and a processor, based on the refrigerationdehydration and absorption technologies, for efficient andcost-effective comprehensive processing of NG. The said processor couldsimultaneously perform the removal of moisture and the recovery of thehigher hydrocarbons (C₂ ⁺) in a single piece of equipment, thussubstantially reducing the capital and operational costs of the NGprocessing plant.

Another objective of the present invention is to provide anenergy-saving comprehensive NG process and a processor that, whenprocessing high pressure NG, does not need external energy forrefrigeration.

A further objective of the present invention is to provide ahigh-efficiency free-piston expander-compressor to provide the requiredrefrigeration.

SUMMARY OF INVENTION

With regard to the above and other objectives, the present inventionprovides a comprehensive NG process and a processor to simultaneouslyperform refrigeration dehydration and refrigeration absorption of higherhydrocarbon gases with maximum recovery rate at minimum energyconsumption. The final product is a gaseous mixture enriched in higherhydrocarbons with minimum residual methane.

The said apparatus comprises the following major components: anintegrated NG processor (hereafter abbreviated as “processor) with arefrigeration dehydration section (hereafter abbreviated as“dehydrator”) and a refrigeration absorption section (hereafterabbreviated as “absorber”); a heat-transport medium (hereafterabbreviated as “medium”) cooler; an absorbent cooler; a fractionaldistiller; a gas-hydrate inhibitor (hereafter abbreviated as“inhibitor”) regenerator; and a refrigeration unit.

The principle of the operations of the comprehensive NG processorfollows. The inlet moisture-laden NG, flowing upward from the bottom ofthe dehydrator, is cooled down to the desired dewpoint temperature bydirectly contacting a down-flowing, adequately dispersed low-temperaturemedium stream. The medium is an aqueous solution containing aninhibitor. The moisture in the inlet NG is condensed on the surface ofthe medium droplets. The medium, diluted with the condensates, isre-concentrated in an inhibitor regenerator and recycled. The dehydratedNG continues to flow upward into the absorber wherein the higherhydrocarbon gases are absorbed with a down-flowing, adequately dispersedlow-temperature absorbent (e.g., heavy oil) stream. The light oil-ladenabsorbent (hereafter abbreviated as “rich oil”) then enters thefractional distiller wherein the absorbed higher hydrocarbons isseparated as the final product. The recovered absorbent is cooled in theabsorbent cooler and recycled to the absorber of the processor. Theprocessed NG, basically free from higher hydrocarbons (hereafterabbreviated as “lean NG”), is re-heated and eventually delivered to theNG transportation pipeline. The refrigeration unit provides the requiredrefrigeration for both medium cooler and absorbed cooler.

When the pressure of the inlet NG is sufficiently high, the requiredrefrigeration could be provided with expanding the dehydrated highpressure NG. In such a “self-refrigeration” case, no external energy isrequired.

In case of the pressure difference between the inlet NG and the NGtransportation pipeline is small, a high-efficiency free-piston NGexpander-compressor is proposed in the present invention to provide therequired self-refrigeration.

BRIEF DESCRIPTION OF DRAWINGS

The above and other features and advantages of the present inventionwill now be further described in the following detailed descriptionsection in conjunction with the attached drawings in which:

FIG. 1 illustrates one preferred embodiment of the comprehensive NGprocessorprocessing of the present invention wherein a separateindustrial refrigeration unit is used to provide the requiredrefrigeration.

FIG. 2 illustrates another preferred embodiment of the comprehensive NGprocessorprocessing of the present invention wherein an integrated NGexpander-compressor is used to provide the required self-refrigeration.

FIG. 3 illustrates the high-efficiency free-piston NGexpander-compressor for providing the required self-refrigeration.

DETAILED DESCRIPTION

FIG. 1 illustrates one preferred embodiment of the comprehensive NGprocessorprocessing of the present invention wherein a separateindustrial refrigeration unit is used to provide the requiredrefrigeration.

The said apparatus comprises the following major components: a processor1 comprising a dehydrator 1a and an absorber 1b; a medium cooler 9comprising a pre-cooler 9a and a deep-cooler 9b; an absorbent cooler 25comprising a pre-cooler 25a and a deep-cooler 25b; a fractionaldistiller 27; an inhibitor regenerator 15, and a refrigeration unit 90.

The inlet NG, laden with moisture and all the higher hydrocarboncomponents, i.e., C₂, C₃, C₄, and C₅ ⁺, enters the dehydrator 1a fromthe bottom via the raw NG inlet pipeline 2 and flows upward.

A low-temperature medium, containing an inhibitor, enters from the topof the dehydrator via the medium inlet pipeline 3. The medium isdistributed or dispersed with the medium distributor 4 over the wholecross-section of the dehydrator and flows downward.

The medium is an aqueous solution of an inhibitor, such as an ionic saltor an organic compound. The concentration of the said inhibitor shouldbe sufficient high to prevent the formation of gas-hydrates/ice over theentire temperature range of the dehydrator operations.

The medium is either sprayed as finely divided droplets or is dispersedwith a packed column to provide extensive contacting surfaces forcooling the up-flowing NG. The moisture in the NG condenses on thedispersed medium surfaces and dissolves into the inhibitor solution. Theslightly diluted medium is eventually discharged from the bottom of thedehydrator via the medium discharge pipeline 5.

The discharged medium is re-pressurized with the pump 6. A major portionof the re-pressurized medium passes through the regulation valve 7 andis sent to the primary side of the pre-cooling section 9a of the mediumcooler 9 via the medium transfer pipeline 8.

A small fraction of the re-pressured medium is diverted via the effluenttransfer pipelines 4 into the inhibitor regenerator 15 wherein thediluted inhibitor solution is re-concentrated. The highly concentrateinhibitor solution is sent via the inhibitor recycle pipeline 16 andmixes with the medium flowing in the medium transfer pipeline 8. Thewastewater separated in the regenerator is discharged via the wastewaterdischarge pipeline 17.

In the medium cooler, the medium is first pre-cooled with the cold leanNG reflux coming from the integrated NG processor via the lean NG outletpipeline 23. The re-heated lean NG is delivered via the lean NG deliverypipeline 11 to the NG transportation pipeline (not shown).

The pre-cooled medium continues to flow upward into the primary side ofthe deep-cooler 9b wherein it is deep-cooled to the requiredlow-temperature with the refrigerant (or brine) provided with theindustrial refrigerator 90. The refrigerant enters the secondary side ofthe deep-cooler via the refrigerant inlet pipeline 12 and leaves via therefrigerant outlet pipeline 13. The deep-cooled medium is recycled intothe dehydrator via the medium inlet pipeline 3. The makeup medium isintroduced via the medium makeup pipeline 10.

In case the concentration of the higher hydrocarbons in NG is so highthat the light oil gas partially condenses into liquid in the dehydrator1a. The mixed condensates of water in the medium and light oil iscollected at the bottom of the dehydrator. The light oil layer flowingover the liquid medium is discharged via the light oil outlet 18 as apart of the final product.

Now return to the absorber 1b of the integrated NG processor. Thedewpoint of the dehydrated NG when leaving from the top of thedehydrator is close to the entrance temperature of the deep-cooledmedium. The cold dehydrated NG enters the absorber from the bottom, andflows upward through a series of bypass pipes 19 in the enriched oilcollector 19a. The up-flowing dehydrated NG comes into contact with thedown-flowing cold absorbent running through a packed column 20. A steamof the deep-cooled absorbent enters from the top of the absorber via theabsorbent inlet pipeline 21. The absorbent is distributed by theabsorbent distributor 22. The temperature of the absorbent at the top ofthe absorber is kept slightly about the dewpoint of the dehydrated NG toavoid gas-hydrate formation.

With such a counter-extraction process in the absorber, the recoveryrates of the light oil gases (C₃+) are very high. A reasonable fractionof ethane (C₂) is also recovered. At the same time, the absorption rateof methane is relatively low. As mentioned above, the lean NG leaves thetop of the absorber via the lean NG outlet pipeline 23, and enters thesecondary side of the pre-cooler 9a of the medium cooler 9.

The rich oil flows out from the absorber 1b via the rich oil outletpipeline 24 and enters the secondary side of the pre-cooler 25a. Therich oil absorbs heat from the recycling absorbent flowing in theprimary side of the pre-cooler. The rich oil leaves the pre-cooler viathe rich oil transfer pipeline 26 and enter the fractional distiller 27wherein the final product, a gaseous mixture enriched in higherhydrocarbons, is separated from the absorbent. The separated higherhydrocarbons gas mixture is delivered via the product outlet pipeline 28to a refiner (not shown).

The energy required for the fractional distillation process is providedwith a heating medium entering the distiller via the heat medium inletpipeline 29 and leaving by the heat medium outlet pipeline 30.

The recovered absorbent, leaving the fractional distiller via theabsorbent outlet pipeline 31, is re-pressurized with a pump 32. Theabsorbent enters the primary side of the absorbent cooler 25 via theabsorbent recycle pipeline 310.

The recycled absorbent flows upward through the primary side of theabsorbent cooler 25. It is first pre-cooled with the cold rich oilflowing in the secondary side of the pre-cooler 25a, and thendeep-cooled with the refrigerant flowing in the secondary side of thedeep-cooler 25b. The refrigerant enters the secondary side of theabsorbent deep-cooling section via the refrigerant inlet pipeline 33 andleaves via the outlet pipeline 34. The refrigerant is provided with theindustrial refrigerator 30.

FIG. 2 illustrates another preferred embodiment of the comprehensive NGprocessorprocessing of the present invention, in which an integrated NGexpander-compressor is used to provide the required“self-refrigeration”. The said embodiment is applicable when thepressure of the lean NG is sufficiently higher than the NG pressurerequired in the NG transport pipeline. The lean NG may be expanded inthree different kinds of gas expansion devices.

According to the magnitudes of the pressure difference between inlet NGand the dehydrated NG transportation pipeline, there are three optionsfor the NG expansion devices. (1) When the said pressure difference isquite large, a simple expansion valve could be used to expand the inletNG to a pressure above or equal to the transportation pipeline pressureand obtain the desired low temperature for refrigeration. In this case,the de-pressurized NG needs no re-compression. (2) When the saidpressure difference is moderately high, the inlet NG has to be expandedbelow the transportation pipeline pressure to obtain the desired lowtemperature for refrigeration. A portion of the expansion energy needsto be recovered for re-compression the de-pressurized NG. In this case,a turbo expander-compressor is preferred. (3) When the said pressuredifference is rather small, but still relevant, the expansion energymust be recovered to the maximum extent for NG re-compression. In thiscase, the high efficiency free-piston expander-compressor, as describedin the following FIG. 3, is recommended.

It should be noted, for both cases (2) and (3), an external powered NGcompressor may also be incorporated, as appropriate, for re-compressingthe de-pressurized NG to the required pressure of the NG transportpipeline.

Return to FIG. 2 wherein a turbo NG expander-compressor as mentioned inthe case (2) is illustrated as an example.

Because most components of the comprehensive NG processor in FIG. 2 areidentical to those in FIG. 1, they are labeled with the same numbers inFIG. 2. Only the dissimilar components of the self-refrigeration unitare labeled with different numbers and will be described in detailsbelow. These dissimilar components include the turbo expander 35a andcompressor 35b, the medium cooler 41, and the filter 38.

The lean NG, left the absorber 1b via the lean NG outlet pipeline 23 andmixed with the inhibitor introduced via the inhibitor injection pipeline36, enters the turbo expander 35a and is expanded. Gas expansion causesthe NG temperature sharply dropped to the required low temperature. Asmall amount of the residual moisture is condensed into tinny liquiddroplets entrained in the chilly lean NG. The chilly lean NG enters thefilter 38 via the de-pressurized NG transfer pipeline 37. The liquiddroplets are separated as an effluent, and the latter is discharged intothe inhibitor regenerator 15 via the effluent pipeline 39. The driedchilly lean NG enters the secondary side of the medium cooler 41 via thechilly lean NG inlet pipeline 40. The chilly lean NG absorbs the heatfrom the recycled medium and flows into the compressor 35b via thede-pressurized NG return pipeline 42. A portion of the chilly NG isdiverted via the bypass valve 44 and bypass pipeline 33 to the absorbentcooler 25, and returns via the bypass return pipeline 34. The lean NG isthen re-compressed to the required pressure and delivered via the leanNG delivery pipeline 43 to the NG transportation pipeline (not shown).

As described above, the system in FIG. 2 does not require any externalenergy to provide the self-refrigeration.

Having described the features and the advantages of the variousembodiments of the present invention as a comprehensive NG processingapparatus, it should be pointed out that the dehydration section withits accessories could also be operated independently as a pure NGdehydrator, without incorporating the absorption section and itsaccessories.

FIG. 3 illustrates the high-efficiency free-piston NGexpander-compressor for self-refrigeration.

The light alloy body 45 of the said free piston expander-compressorcomprises two cylinders with different diameters. The smaller cylinder46 is the expander, and the larger cylinder 47 the compressor. Two freepistons, 48 and 49, are rigidly connected with a short hollow shaft 50to form a single integrated moving part. Since the latter is a compact,light-weighted component, very high frequency operation and highmechanical efficiency are feasible. For a high-pressure NG, the size ofsuch a free piston machine is relatively small. For example, for anapparatus processing 500,000 m³ STP per day, under an initial pressureof 10 MPA and an exit pressure of 5 MPA, the maximum diameter of thefree piston expander-compressor will be in the order of 12 cm whenworking at 4,000 strokes per minute.

In FIG. 3, the NG inlet pipelines 51 and 52 and the outlet pipelines 53and 54 of the expander, as well as the inlet pipelines 55 and 56 and theoutlet pipelines 57 and 58 of the compressor are connected to therelevant cylinders as illustrated. The associated valves controllingthese inlet pipelines and outlet pipelines are similar to those used inmodern high-speed internal combustion engine. These valves are not shownin FIG. 3.

In case that the pressure difference between the inlet NG and the outletNG to the pipeline is too small so that additional external compressingenergy is required, a viable option is to connect the said free pistonwith extending the shaft 59, as shown by the dotted line, to aconventional reciprocating piston-type gas engine, not shown in FIG. 3.

In summary, the present invention is related to an apparatus forefficient and cost-effective comprehensive processing of NG, includingthe removal of moisture and the recovery of the higher hydrocarbons (C₂⁺), in a single integrated processing unit. The present inventionprovides a low-cost comprehensive NG processor that is universallyapplicable to both terrestrial and off-shore NG exploitation.

Having describes the present invention and preferable embodimentsthereof, it will be recognized that numerous variations, substitutionsand additions may be made to the present invention by those ordinaryskills without departing from the spirit and scope of the appendedclaims.

1. A comprehensive gas processor for removing the moisture andrecovering the higher hydrocarbons (i.e., C₂ ⁺) therein either on-situin a gas field or in a plant comprising: (a) an integrated gas processorcomprising two sections working on a hybrid process, i.e., anintegration of two different processes within a single casing; i) arefrigeration-dehydration section working on refrigeration processwherein the inlet gas contacts with a counter-flowing stream ofdispersed cold heat-transport medium containing a non- or low-volatilehydrate inhibitor with boiling point higher than 180° C. and themoisture of said gas is condensed and removed with the coldheat-transport medium; and ii) an absorption section working onlow-temperature absorption process wherein the dehydrated gas contactswith a counter-flowing stream of dispersed liquid absorbent with ahigher hydrocarbon gas solubility higher greater than 20 scf/gal whereinthe higher hydrocarbons (i.e., C₂ ⁺) are absorbed. under said absorptionconditions; (b) a heat-transport medium cooler comprising a pre-coolingstage and a deep-cooling stage wherein in said pre-cooling stage saidheat-transport medium is pre-cooled with the cold outlet gas left saidintegrated gas processor and in said deep-cooling stage the medium isdeep-cooled with the refrigerant provided with a refrigerator; (c) anabsorbent cooler comprising a pre-cooling stage and a deep-cooling stagewherein in said pre-cooling stage said recycling absorbent is pre-cooledwith the cold outlet absorbent left said integrated gas processor and insaid deep-cooling stage the absorbent is deep-cooled with therefrigerant provided with a refrigerator; (d) a fractional distiller forseparating the absorbed higher hydrocarbons as a product from the outletabsorbent left said integrated gas processor and then the separatedabsorbent is recycled back to said integrated gas processor; (e) aninhibitor regenerator for concentrating the low-volatile hydrateinhibitor to be recycled and discharging the wastewater; (f) arefrigerator for providing the refrigerant to said deep-cooling stagesof said heat-transport medium cooler and said absorbent cooler; (g) apipeline for delivering the recovered higher hydrocarbons; and (h) a gasinlet pipeline and a pipeline for delivering the processing gas.
 2. Acomprehensive gas processor of claim 1 wherein the dehydration sectionof said integrated processor and its accessories (comprising saidheat-transport medium cooler, said inhibitor regenerator, saidrefrigerator, and said gas inlet-pipeline and a pipeline for deliveringthe processed gas) are operated independently as a gas dehydratorwithout incorporating the absorption section.
 3. A comprehensive gasprocessor of claim 1 wherein said heat-transport medium is an aqueoussolution of calcium chloride or other ionizing salts and theregeneration rate of said solution is less than 5 liter per kg ofwastewater to be discharged .
 4. A comprehensive gas processor of claim1 wherein said heat-transport medium is an aqueous solution of ethyleneglycol or other organic compounds with boiling points higher than 180°C. and the regeneration rate of said solution is less than 5 liter perkg of wastewater discharged.
 5. A comprehensive gas processor of claim 1wherein said absorbent is heavy oil (i.e., hydrocarbon mixture withmolecular weight higher than 100) or other organic compounds withhydrocarbon gas solubility higher than 20 scf/gal liquid.
 6. Acomprehensive gas processor of claim 1 when working on inlet gaspressure greater than 5.0 MPa wherein said refrigerant to saiddeep-cooling stages of said heat-transport medium cooler and saidabsorbent cooler is provided with a gas expansion device with the inletgas pressure is greater than 5.0 MPa.
 7. A gas expansion device of claim6 wherein said expansion device is a triple-sectional free-piston gasexpander-compressor-booster comprising: (a) a gas expansion cylinder anda gas compression cylinder; (b) a co-shaft gas expansion piston and gascompression piston; and (c) a co-shaft gas-fueled booster piston-engineproviding supplemental power for compressing said expanded gas to therequired delivery pipeline pressure.
 8. A continuous process forseparating moisture and hydrocarbons higher than methane from a naturalgas stream at pipeline or wellhead pressure, comprising removing saidmoisture and said hydrocarbons heavier than methane as a C₂ ⁺ mixture,including the following steps: (a) Cooling said natural gas stream bydirectly contacting the natural gas stream with a low temperaturecounter-flowing heat-transport medium containing an aqueous solution ofa gas-hydrate inhibitor at pipeline or well-head pressure and at a ratesufficient to cool said gas to the low temperature required by theabsorption Step (c), and the majority of water moisture is condensed anddissolved into the heat-transport medium and, at the same time, aportion of higher hydrocarbons is also condensed as a liquid that isinsoluble to the heat transport medium (b) Separating the insolublecondensed higher hydrocarbons liquid from the condensed water (alreadydissolved into the heat-transport medium) as a portion of the productstream; (c) Extracting said dehydrated, depleted cold natural gas streamleaving Step (a) by flowing counter-flow to the cold natural gas streama cold absorbent with a higher hydrocarbon (i.e., C ₂ ⁺) solubilitygreater than 20 scf/gal under said absorption conditions and at a ratesufficient to produce a rich absorbent stream containing C ₂ ⁺ mixtureand a small portion of methane, and a cold residue natural gas stream ofgas transport pipeline quality; (d) Regenerating said rich absorbent byfractionating the rich absorbent leaving Step (c) at reduced pressureand separating from said absorbent the absorbed C ₂ ⁺ mixture as aproduct stream; (e) Cooling the heat-transport medium leaving Step (a)to said sufficiently low temperature in two steps: first with the coldstream of the residue gas leaving Step (c), then with the refrigerantprovided by an external refrigerator; (f) Separating the condensed watermoisture from said heat transport medium by evaporating a small portionof the heat-transport medium leaving Step (a) under reduced pressure,and recycling the concentrated inhibitor solution to the heat-transportmedium stream; (g) Recycling the regenerated absorbent leaving Step (d)by compressing it to pipeline or well-head pressure, and cooling it tothe required low temperature in two steps: first with the rich absorbentleaving Step (c), then with the refrigerant provided by an externalrefrigerator; and (h) Delivering the residue gas leaving Step (e) intothe gas transport pipeline.
 9. The process of claim 8, wherein the coldstream of the residue gas leaving Step (c) is expanded to a lowerpressure and much lower temperature to provide sufficient internalrefrigeration.
 10. The process of claim 9, wherein in Step (e) theheat-transport medium leaving Step (a) is cooled to said sufficientlylow temperature with the cold stream of the said expanded residue gas.11. The process of claim 8, wherein in step (a) the said heat transportmedium is an aqueous solution of calcium chloride or other ionizingsalts with a potential regeneration rate less than 5 liter per kg ofwastewater to be discharged.
 12. The process of claim 8, wherein in step(a) the said heat transport medium is an aqueous solution of ethyleneglycol or other organic compounds with a potential regeneration rateless than 5 liter per kg of wastewater to be discharged.